Natural gas, which primarily includes methane, has grown into a viable alternative energy source to petroleum over recent years, especially in the United States, due to drastic increases in proven worldwide and domestic reserves and due to a desire for increased energy independence. Many proven natural gas reserves, however, are characterized as sub-quality due to the presence of compounds other than methane therein. While high-quality natural gas reserves may require less processing for commercialization, the sub-quality natural gas reserves are generally significantly cheaper sources of natural gas. In addition, the sub-quality natural gas reserves provide cost-savings opportunities as more efficient processing techniques are developed to process the natural gas from the reserves for commercialization.
One processing consideration for commercializing natural gas involves liquifying the natural gas, which provides ease of storage and transport and which can decrease a volume of the natural gas by up to 600 times. High-quality natural gas reserves may be liquified with relative ease. However, difficulties with liquifying natural gas from sub-quality natural gas reserves persist due to the presence of compounds other than methane. In particular, compounds that freeze at higher temperatures than a boiling point of methane may be present in the sub-quality natural gas reserves and may freeze during liquefaction of the natural gas, thereby causing plugging and blockage in pipes during liquefaction. Examples of compounds that may be present in the natural gas and that may freeze during liquefaction include benzene, toluene, xylene, cyclohexane, and neopentane. Neopentane is particularly problematic due to its high freezing point of about −17° C., which will generally result in freezing during liquefaction of the natural gas, and due to its lower molecular weight and unique spherical molecular structure compared to benzene, toluene, and xylene, which makes neopentane more difficult to separate from the natural gas than benzene, toluene, and xylene.
Adsorption methods have been developed for selectively removing compounds from natural gas in preparation for liquefaction. Adsorption generally involves collection of molecules on a surface of an adsorbent. For example, silica gels, aluminosilicate gels, zeolite molecular sieves, and activated carbon are known adsorbents for adsorbing various compounds from natural gas. Due to relative ease of regeneration as compared to other adsorbents, silica gels and aluminosilicate gels have gained widespread use for depleting natural gas of various hydrocarbons such as benzene, toluene, xylene, and other hydrocarbons having greater than 8 carbon atoms. However, to deplete the natural gas of certain hydrocarbons such as C5 to C7 hydrocarbons including heptanes, cyclohexanes, benzene, and neopentane to desirable concentrations, adsorbent beds including the silica gels and aluminosilicate gels are generally required to have a higher volume than would otherwise be required to deplete the natural gas of C8 or greater hydrocarbons alone.
Despite the benefits associated with adsorbing compounds from natural gas using silica gels and aluminosilicate gels, it is desirable to maximize efficiency of C8 or greater hydrocarbon adsorption, in addition to maximizing efficiency of C5 to C7 hydrocarbon adsorption, especially neopentane, cyclohexane, benzene, and heptane adsorption, from natural gas to minimize a concentration of the aforementioned hydrocarbons in the natural gas feed into permissible ranges for liquefaction. It is also desirable to minimize adsorbent bed volume while avoiding excessive regeneration requirements for adsorbents that are employed in the adsorbent beds.